Recently, along with rapid development of information-related equipments and communications equipments such as personal computers and mobile phones, a demand for a non-aqueous electrolyte secondary battery such as a lithium secondary battery which is small in size and light in weight and which has a high energy density, has been high. As a cathode active material for the non-aqueous electrolyte secondary battery, a composite oxide of lithium and a transition metal, such as LiCoO2 LiNiO2 LiNi0.8Co0.2O2 and LiMn2O4 has been known
Among others, the lithium secondary battery using the lithium cobalt composite oxide (LiCoO2) as the cathode active material and using a lithium alloy or carbon such as graphite or carbon fiber as a negative electrode, provides a high voltage at a level of 4 V, and is thus widely being used, particularly, as a battery having a high energy density. However, there is a problem that a raw material compound as a cobalt source for the lithium cobalt composite oxide is rare and expensive.
On the other hand, the lithium nickel composite oxide (LiNiO2) using relatively inexpensive nickel provides a high capacity, but has a problem that its thermal stability is low and the safety in use as a battery is lower than that in use of the lithium cobalt composite oxide (LiCoO2). Furthermore, the lithium manganese composite oxide (LiMn2O4) with a spinel structure using inexpensive manganese provides high thermal stability and high safety in use as a battery, but has a problem that its capacity is low.
Under such circumstances, attention is attracted to cathode active materials such as lithium nickel manganese (Li—Ni—Mn) composite oxide, lithium nickel cobalt (Li—Ni—Co) composite oxide and lithium nickel manganese cobalt (Li—Ni—Mn—Co) composite oxide which make up for the drawbacks in single use of cobalt, nickel or manganese element and have the advantages. However, none of these cathode active materials containing at least two transition metal elements succeeded in satisfying all the properties including the discharge capacity, the cyclic charge and discharge properties relating to reduction of the discharge capacity caused by repetitive charge and discharge cycles, a rate property relating to a capacitance available for discharge in a short period of time, and the thermal stability in heating durations after charging (which will also be referred to simply as “safety” in the present specification).
Techniques below are known for solving these problems. For example, a proposed compound is an Li—Ni—Mn—Co—Al composite oxide obtained by mixing a lithium compound, a nickel compound, a cobalt compound and a manganese compound, and further adding an aluminum compound and by firing the mixture (cf. Patent Document 1 and Patent Document 2).
Another proposed compound is a cathode active material, a particle surface of which is coated with aluminum, obtained by dispersing and stirring LiMn0.4Ni0.4Co0.2O2 or Li1.1Mn0.31Ni0.38Co0.31O2 synthesized by a solid-phase method, in an isopropyl alcohol solution of Al(OC3H7)3 and thermally treating the mixture at 600° C. Still another proposed compound is a cathode active material, a particle surface of which is coated with aluminum, obtained by dispersing and stirring Li1.05Mn0.3Ni0.7O2 in an aqueous solution of Al(CH3COCHCOCH3)3 and thermally treating the mixture at 500° C. (cf. Patent Document 3).
Furthermore, the following lithium-containing composite oxide is proposed: first, a co-precipitated composite hydroxide containing nickel and cobalt, or nickel, cobalt and manganese is mixed with lithium hydroxide monohydrate, and the mixture is fired to synthesize a lithium-containing composite oxide with a composition of Li1.08Ni0.7Co0.3O2 or Li1.08Ni0.34Co0.33Mn0.33O2. The lithium-containing composite oxide and powdery aluminum metal are added in water to obtain a slurry, and the slurry is further stirred to dissolve the aluminum metal, and then dried at 80° C., thereby obtaining a lithium-containing composite oxide, a surface of which is covered with a layer containing aluminum hydroxide, aluminum oxide and lithium carbonate (cf. Patent Document 4).
The following composite oxide is also proposed: lithium carbonate, manganese dioxide, nickel oxide and cobalt oxide are mixed and fired to synthesize a lithium-containing composite oxide with a composition of any one of LiMn0.4Ni0.4Co0.2O2 Li1.1Mn0.3Ni0.6Co0.1O2 and Li1.1Mn0.25Ni0.45Co0.3O2. Then aluminum stearate is added to the synthesized lithium-containing composite oxide, and the mixture is mixed and crushed with a ball mill and subjected to a heat treatment at 600° C., thereby obtaining a lithium-containing composite oxide, a particle surface of which is modified with an aluminum compound (cf. Patent Document 5).
Patent Document 1: JP-A-9-237631
Patent Document 2: JP-A-2003-151548
Patent Document 3: JP-A-2005-310744
Patent Document 4: JP-A-2005-322616
Patent Document 5: JP-A-2005-346956